Synchronous Optical Network (SONET) and Optical Transport Network (OTN) define data transmission formats. A data stream can be conveyed through one or more physical networks and routed among multiple network paths by formatting electrical and/or optical signals as SONET or OTN data. Both formats package the signals into frame structures. Each frame includes a certain amount of control data, or overhead data, that is used to negotiate the frame through the physical network(s). Each frame also includes a certain amount of payload data that comprises the actual information that is being conveyed.
Fiber optics have provided a substantial increase in the volume of data that networks can carry over long distances. SONET and OTN define a number of different data rates for optical networks, currently from about 52 Megabits per second to about 40 Gigabits per second over a single optical fiber. In comparison, a typical analog modem operating at maximum efficiency can achieve a mere 56 Kilobits per second. At the 40 Gigabit rate, a single optical fiber can carry enough data to handle over 400,000 simultaneous voice calls. Optical networks are likely to carry even larger volumes of data in the future.
SONET was developed and adopted several years ago as a standardized data format for optical networks. At the time, data rates were primarily limited by the speed at which electrical components at either end of an optical fiber could process electrical signals. That is, even in an optical network, the data is still usually routed and processed in electrical form. The optical fibers are basically just pipes through which pulses of light are sent. A data stream is usually converted between electrical and optical signals each time the data stream is forwarded or routed through another segment of optical fiber.
The data rates for processing electrical signals have increased dramatically since the early days of optical networking. At the 40 Gigabit rate and beyond, new challenges have come to light that were not addressed by SONET. For instance, optical fibers are “lossy.” That is, an optical signal loses intensity gradually as light “leaks” out over a length of fiber. If the signal intensity drops too low, the data represented by the signal cannot be recovered. In which case, optical signals are regenerated at particular intervals to maintain integrity of the data. Furthermore, at higher data rates, pulses of optical light are shorter than at lower data rates. Shorter pulses are more difficult to recognize than longer pulses. So, an amount of signal degradation that is acceptable at a lower data rate may not be acceptable at a higher data rate. In which case, the intervals at which optical signals are regenerated may be short enough to maintain signal integrity at lower data rates but not at higher data rates.
Obviously, one solution to improve signal integrity for higher data rates is to use shorter intervals of optical fiber. However, shortening the intervals by adding regeneration hardware to physical networks on a nation-wide or global scale would be prohibitively expensive. OTN provides an alternative solution to this problem, among other things.
In OTN, when a data stream is packaged into frames, forward error correction data is calculated for groups of payload data and then appended to a frame. When the payload data is extracted from the frame, the forward error correction data is used to detect and correct errors in the payload. The error correction data adds somewhat more overhead to the data stream than SONET overhead, but by solving the signal integrity issue, much higher data rates are possible over longer distances.